Field of the Invention
The present invention relates to pressure detectors of a type utilizing a sensor chip as a sensor element or a strain gauge; such pressure detectors being mainly used to detect pressures of highly corrosive gas lines, and the like, in semiconductor manufacturing processes.
Heretofore, pressure detectors using sensor chips (as pressure-sensor elements) or strain gauges have been widely utilized to detect fluid pressures inside pipe lines, and the like.
FIG. 7 shows an example of such a pressure detector A, which includes a stainless-steel (SUS 316L) sensor base 1, a sensor chip (a pressure sensor or a semiconductor strain gauge) 2, a diaphragm (SUS 316L) 3, silicone oil 5, a ball 6, a lead pin 7, and the like.
In the pressure detector A in FIG. 7, an outer peripheral edge of the diaphragm 3 (formed of stainless-steel (SUS 316L), having a thickness of about 50 .mu.m, has been welded to a lower end face of the sensor base 1 by means of laser welding, and, when a fluid pressure B applied to a gas-contact face 3a of the diaphragm 3 is transmitted through the silicon oil 5 to the sensor chip 2, values of four strain resistors formed on the sensor chip 2 are changed to cause a signal proportional to the received pressure B. This signal is output at an output terminal of a bridge circuit composed of the four strain resistors.
Further, reference number 16 designates a weld of the diaphragm; and reference number 10 designates a weld of the ball 6.
The above-mentioned pressure detector of FIG. 7 can detect fluid pressure inside a pipe line with a high accuracy and a relatively high sensitivity, thus having excellent and practical usefulness.
However, many problems which must be solved have remained in this pressure detector, the most important of which is corrosion, such as catalytic activity, or the like, on the gas-contact face 3a of the diaphragm 3.
That is, in the pressure detector of FIG. 7, the diaphragm 3 is formed of a stainless-steel (SUS 316L), and a mixed oxide layer of Fe and Cr, having a thickness of about 30 .ANG., is formed on an electropolished outer face layer of the gas-contact face 3a.
However, for a highly corrosive gas such as a halogen based gas including hydrogen chloride (HCl) and hydrogen bromide (HBr) and a fluorine based gas including fluorine gas (F.sub.2) and hydrogen fluoride (HF), which are used for semiconductor manufacturing processes, corrosion significantly progresses due to water, and the like, contained in the gas itself, thereby causing a, so-called, metallic contamination.
Further, in a pipe line for semiconductor manufacturing, an outgas from the contact face 3a and catalytic activation as well as the corrosiveness of the gas-contact face 3a of the above-mentioned diaphragm are also serious problems, so that the gas-contact face 3a is required to be free of gasout, non-catalytic, and non-corrosive.
However, the mixed oxide layer of Fe and Cr formed on the outer face layer of the gas-contact face 3a of the above-mentioned diaphragm 3 is not sufficiently effective to prevent the outgas, causing a large amount of outgas to be released; and, in addition, due to its catalytic action, a self-decomposition of special gases for semiconductor manufacturing is accelerated, whereby various problems arise such as deteriorated product quality.
For this reason, where a pressure detector having a construction as shown in FIG. 7 is used to detect pressure in a pipe line handling a highly corrosive gas, such as the above-mentioned halogen based and fluorine based gases, it is required that the diaphragm 3 have on its gas-contact face 3a a so-called chrome oxide passive-state film (Cr.sub.2 O.sub.3, for halogen based gases) composed of only Cr, not containing Fe, having a thickness of hundreds of .ANG., or a fluoride passive-state film (CrF.sub.3,CrF.sub.2, FeF.sub.2, FeF.sub.3, and the like, for fluorine based gases), or a mixed-oxide passive-state film (Al.sub.2 O.sub.3 /Cr.sub.2 O.sub.3 for ozone gas) composed of mainly aluminum oxide and chrome oxide, thereby protecting the gas-contact face 3a.
This is because: the above-mentioned chrome oxide passive-state film is excellent in corrosion resistance to a halogen-based, highly corrosive, gas, in prevention of outgas, and in non-catalyst properties; the fluoride passive-state film has a high corrosion resistance to a fluorine based, highly corrosive gas and ozone and is excellent in prevention of outgas, and in non-catalyst properties; and further, the mixed-oxide passive-state film composed of mainly aluminum oxide and chrome oxide is excellent in corrosion resistance to an ozone gas having a very strong oxidizing ability.
In order to form the above-mentioned passive-state film composed of 100% chrome oxide, for the diaphragm 3 made of austinitic stainless steel (for example, SUS 316L), it is necessary: (1) to polish the diaphragm 3 using a lapping-polishing method, or the like, so as to allow the outer face to have a microcrystalline structure (a so-called veilubi layer); and (2) heat treat (at 400.degree. C. to 500.degree. C. for one to ten hours) the diaphragm 3 with oxidizing species containing a very small amount of water in a highly reducing atmosphere.
In the same manner, in order to form the fluoride passive-state film, it is necessary: (1) to perform a treatment for forming the passive-state film at 200.degree. C. to 250.degree. C. for one to ten hours in a fluorine gas atmosphere; and (2) to perform a treatment for annealing the passive-state film at 350.degree. C. to 400.degree. C. for one to ten hours.
To form the mixed-oxide passive-state film composed of aluminum oxide and chrome oxide, it is necessary to heat treat (at 400.degree. C. to 600.degree. C. for one to ten hours) the diaphragm 3 of stainless steel containing about 4% of aluminum with oxidizing species containing a very small amount of water in a highly reducing atmosphere.
However, because a sensor chip (a pressure sensor) has a heat resisting temperature of about 150.degree. C. for the conventional pressure detector shown in FIG. 7, the pressure detector cannot be heat treated at these otherwise desirable, high temperatures.
Further, the sensor base 1 and the diaphragm 3 have already been welded in the conventional pressure detector, so that it is difficult to uniformly polish the gas-contact face 3a of the diaphragm 3 at a surface smoothness that a maximum projection value is about 0.7 .mu.m or less, and thus the degree of polishing of the gas-contact face 3a of the diaphragm 3 is hardly uniform.
Still further, for the conventional pressure detector, the weld 16 is positioned on the gas-contact face 3a of the diaphragm 3, so that polishing of the weld 16 brings about a state different from that of non-welded portions, and thus a uniform polished-finish of the weld 16 becomes difficult to achieve.
As a result, for the conventional pressure detector, it is difficult to form the passive-state film, composed of 100% chrome oxide, or the fluoride passive-state film on the gas-contact face 3a of the diaphragm 3, so that as a degree of integration of semiconductors increases, various problems arise such as generation of metallic particles caused by corrosion of the gas-contact face 3a of the pressure detector and/or water content release from the diaphragm face; or troubles caused by catalytic action of the diaphragm face, and thus an improvement of product quality becomes difficult to achieve.
Problems Solved by the Invention
It is an object of this invention to solve the problems described above for conventional pressure detectors; that is, because passive-state films cannot be formed on gas-contact faces of diaphragms of pressure detectors, it is impossible to prevent corrosion of the gas-contact faces of the diaphragms, catalytic action of the diaphragm faces, and water content release from the diaphragm faces. It is a further object of this invention to provide a pressure detector which will not compromise product quality, even when it is used in a semiconductor manufacturing process, in that it provides a diaphragm of a pressure detector with a chrome oxide passive-state film, or a fluoride passive-state film, or a mixed-oxide passive-state film composed mainly of aluminum oxide and chrome oxide on its gas-contact face.